Fix empty group ordering

[waynemwashuma]

Objective

Fix scheduling behavior for empty system groups by treating them as explicit ordering barriers during graph expansion. This ensures ordering constraints remain valid even when intermediary groups contain no systems.

Solution

Previously, group ordering constraints were expanded directly into system-to-system edges. This worked when groups contained systems, but failed for empty groups because they produced no graph nodes during expansion.

As a result, ordering relationships that depended on empty intermediary phases could silently disappear. This introduces explicit graph nodes for system groups and uses them as ordering barriers when a group has no systems.The new approach preserves ordering semantics regardless of whether a group currently contains systems.Instead of collapsing groups prematurely into systems, the scheduler now treats groups as first-class ordering nodes. This allows:

• empty groups to participate in ordering
• future extensibility for other barriers
• consistent graph semantics across systems and groups

Showcase

Before

builder.addGroup({
  label: StartPhase,
  schedule: 'update',
  before: [MiddlePhase]
})

builder.addGroup({
  label: MiddlePhase,
  schedule: 'update',
  before: [EndPhase]
})

builder.addGroup({
  label: EndPhase,
  schedule: 'update'
})

If MiddlePhase contained no systems, the ordering relationship could collapse during graph expansion.

After

builder.addGroup({
  label: StartPhase,
  schedule: 'update',
  before: [MiddlePhase]
})

builder.addGroup({
  label: MiddlePhase,
  schedule: 'update',
  before: [EndPhase]
})

builder.addGroup({
  label: EndPhase,
  schedule: 'update'
})

MiddlePhase now acts as a stable ordering barrier even without systems.

Migration guide

No migration required.

Notes

• Empty system groups now participate in scheduling order resolution
• Existing schedules continue to work unchanged
• Topological sorting still outputs executable systems only

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.